Light bar is a key component in a backlight module of a liquid crystal display (LCD). Multi-string light bar is the most commonly used in an LCD backlight module at present. A plurality of light emitting diodes (LED) is regularly distributed on the bar base to make up a light bar. The existing light bars are usually symmetrically mounted on both the upper and lower sides or on both the left and right sides of the LCD display, or even on all four sides of the LCD display.
However, in a boost converter of a backlight driver, no over power protection circuit is arranged for each string of light bar. There is merely an over current or over voltage protection circuit for the total input. In this case, a few risks can be posed. As shown in FIGS. 1a and 1b, if breakdown occurs to a MOSFET (FIG. 1a) or a BJT (FIG. 1b), the current through the light bar will be out of control, leading to overly large current through the damaged string. Meanwhile, since the currents in present settings are slightly larger than the normal operating current, thus, abnormality to only one string will lead to failure of detecting the above mentioned problem.
In addition, the over power in the string of light bar can result in a variety of problems, such as over brightness in this area, over temperature of the light bar, and burnout of the LCD diaphragm, etc.
Therefore, it is necessary to design a suitable over power protection for the circuit of each string of light emitting diodes.